Apparatuses and methods for accessing memory cells

ABSTRACT

Apparatuses and methods for accessing a memory cell are described. An example apparatus includes a first voltage circuit coupled to a node and is configured to provide a first voltage to the node and includes a second voltage circuit coupled to a node and is configured to provide a second voltage to the node. A memory cell is coupled to first and second access lines. A decoder circuit is coupled to the node and the first access line, and is configured to selectively couple the first access line to the node. The first voltage circuit is configured to provide the first voltage to the node before the second voltage circuit provides the second voltage to the node, and the second voltage circuit stops providing the second voltage before the node reaches the second voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/682,228, filed Aug. 21, 2017, which is a continuation of U.S. patentapplication Ser. No. 15/003,498, filed Jan. 21, 2016, U.S. Pat. No.9,767,896 issued on Sep. 19, 2017. These applications and patent areincorporated by reference herein in their entirety and for all purposes.

BACKGROUND

Semiconductor memories may include threshold-type memory cells, whichare memory cells that are accessed by providing a voltage across amemory cell, and the data value stored is based on a threshold voltageof the memory cell. For example, the data value may be based on whetherthe threshold voltage of the memory cell is exceeded when a voltage isapplied across the memory cell, and whether the memory cell conductscurrent in response to the voltage provided across the memory cell. Thedata value stored may be changed, for example, by applying a voltagesufficient to change the threshold voltage of the memory cell. Anexample of a threshold-type memory cell may be a phase change memorycell.

The voltage applied across a memory cell is typically provided by twoaccess lines to which the memory cell is coupled. Also coupled to eachof the access lines are other memory cells. Each of the memory cells mayhave a respective threshold voltage, with some having a relatively lowthreshold voltage and others having a relatively high threshold voltage.When a voltage is applied to a target memory cell by the two accesslines, the other memory cells coupled to the respective access lines arealso subjected a respective voltage. While the respective voltage towhich the other memory cells is subject is not sufficient to access theother memory cells also coupled to the respective lines, the voltage maynonetheless be sufficiently high to inadvertently degrade or change thethreshold voltage of some of the other cells. For example, where one ofthe other memory cells has a relatively low threshold voltage, and thetarget memory cell has a relatively high threshold voltage, the voltageto which the other memory cell is subjected when accessing the targetmemory cell may be sufficient to degrade the relatively low thresholdvoltage (e.g., increase the relatively low threshold voltage), or changethe relatively low threshold voltage to a relatively high thresholdvoltage, thereby changing the data value stored by the other memorycell. If such an event were to occur, the memory has failed.

Thus, there is a desire to have apparatuses and methods for accessingmemory cells that mitigate application of access voltages that maydegrade performance of the memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a memory according to an embodiment of theinvention.

FIG. 1B is a diagram of a portion of a three-dimensional (3D)cross-point memory array according to an embodiment of the invention.

FIG. 2 is a schematic diagram of a portion of the memory according to anembodiment of the invention.

FIG. 3 is a timing diagram of various signals during a read operationaccording to an embodiment of the invention.

FIG. 4 is a timing diagram of various signals during a read operationaccording to an embodiment of the invention.

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficientunderstanding of embodiments of the invention. However, it will be clearto one skilled in the art that embodiments of the invention may bepracticed without these particular details. Moreover, the particularembodiments of the present invention described herein are provided byway of example and should not be used to limit the scope of theinvention to these particular embodiments. In other instances,well-known circuits, control signals, timing protocols, and softwareoperations have not been shown in detail in order to avoid unnecessarilyobscuring the invention.

FIG. 1A illustrates an apparatus that includes a memory device 104according to an embodiment of the present invention. The memory device104 includes a memory array 160 with a plurality of memory cells thatare configured to store data. The memory cells may be accessed in thearray through the use of various access lines, for example, word lines(WLs) and/or bit lines (BLs). The memory cells may be non-volatilememory cells, such as NAND or NOR flash cells, phase change memorycells, memory cells that include chalcogenide material, or may generallybe any type of memory cells. The memory cells of the memory array 160can be arranged in a memory array architecture. For example, in oneembodiment, the memory cells are arranged in a 3D cross-pointarchitecture. In other embodiments, other memory array architectures maybe used, for example, a single-level cross-point architecture, a 3Darray architecture including a plurality of decks, with each of thedecks including access lines and memory cells, among others. The memorycells may be single level cells configured to store data for one bit ofdata. The memory cells may also be multi-level cells configured to storedata for more than one bit of data. It will be appreciated that those ofordinary skill in the art will have sufficient understanding from thedescription provided herein to modify circuits of a memory device (e.g.,decoder circuits, input/output circuits, control logic, etc.) topractice the disclosed invention, including practicing embodiments ofthe disclosed invention in memory array architectures other than thoseexpressly described herein.

A data strobe signal DQS may be transmitted through a data strobe bus(not shown). The DQS signal may be used to provide timing informationfor the transfer of data to the memory device 104 or from the memorydevice 104. The I/O bus 128 is connected to an I/O control circuit 120that routes data signals, address information signals, and other signalsbetween the I/O bus 128 and an internal data bus 122, an internaladdress bus 124, and/or an internal command bus 126. The internaladdress bus 124 may be provided address information by the I/O controlcircuit 120. The internal address bus 124 may provide block-row addresssignals to a row decoder 140 and column address signals to a columndecoder 150. The row decoder 140 and column decoder 150 may be used toselect blocks of memory cells for memory operations, for example, readand write operations. The row decoder 140 and/or the column decoder 150may include one or more signal line drivers configured to provide abiasing signal to one or more of the signal lines in the memory array160. The I/O control circuit 120 is coupled to a status register 134through a status register bus 132. Status bits stored by the statusregister 134 may be provided by the I/O control circuit 120 responsiveto a read status command provided to the memory device 104. The statusbits may have respective values to indicate a status condition ofvarious aspects of the memory and its operation.

The memory device 104 also includes a control logic 110 that receives anumber of control signals 138 either externally or through the commandbus 126 to control the operation of the memory device 104. The controlsignals 138 may be implemented with any appropriate interface protocol.For example, the control signals 138 may be pin based, as is common indynamic random access memory and flash memory (e.g., NAND flash), orop-code based. Example control signals 138 include clock signals,read/write signals, clock enable signals, etc. A command register 136 iscoupled to the internal command bus 126 to store information received bythe I/O control circuit 120 and provide the information to the controllogic 110. The control logic 110 may further access a status register134 through the status register bus 132, for example, to update thestatus bits as status conditions change. The control logic 110 may beconfigured to provide internal control signals to various circuits ofthe memory device 104. For example, responsive to receiving a memoryaccess command (e.g., read, write), the control logic 110 may provideinternal control signals to control various memory access circuits toperform a memory access operation. The various memory access circuitsare used during the memory access operation, and may generally includecircuits such as row and column decoders, charge pump circuits, signalline drivers, data and cache registers, I/O circuits, as well as others.

A data I/O circuit 170 includes one or more circuits configured tofacilitate data transfer between the I/O control circuit 120 and thememory array 160 based on signals received from the control logic 110.In various embodiments, the data I/O circuit 170 may include one or moreregisters, buffers, and other circuits for managing data transferbetween the memory array 160 and the I/O control circuit 120. Forexample, during a write operation, the I/O control circuit 120 receivesthe data to be written through the I/O bus 128 and provides the data tothe data I/O circuit 170 via the internal data bus 122. The data I/Ocircuit 170 writes the data to the memory array 160 based on controlsignals provided by the control logic 110 at a location specified by therow decoder 140 and the column decoder 150. During a read operation, thedata I/O circuit reads data from the memory array 160 based on controlsignals provided by the control logic 110 at an address specified by therow decoder 140 and the column decoder 150. The data I/O circuitprovides the read data to the I/O control circuit via the internal databus 122. The I/O control circuit 120 then provides the read data on theI/O bus 128.

FIG. 1B is a diagram of a portion of a 3D cross-point memory array,generally designated 180, in accordance with an embodiment of thepresent invention. In various embodiments, the memory array 180 can beimplemented as the memory array 160 of FIG. 1A. The memory array 180includes a first number of access lines 182A, 182B, . . . , 182N(collectively referred to as access lines 182) and a first number ofaccess lines 186A, 186B, . . . , 186N (collectively referred to asaccess lines 186). As shown in FIG. 1B, the access lines 182 may bearranged parallel to one another. The access lines 186 can be arrangedparallel to one another and orthogonal to the access lines 182. Theaccess lines 182 and the access lines 186 can be made from a conductivematerial, such as copper, tungsten, titanium, aluminum, etc. Layers ordecks of access lines can be stacked to create a 3D lattice structure.As shown in FIG. 1B, layers of access lines 182 alternate with layers ofaccess lines 186 to form a 3D structure.

The memory array 180 includes a plurality of memory cells 184. In oneembodiment, the memory cells 184 can be phase change memory cells. Insome embodiments, the memory cells 184 may include a chalcogenidematerial. Each memory cell 184 is connected to first and second accesslines (e.g., access line 182A and access line 186A). The layers or decksof access lines may be separately addressed so that memory cellsassociated with the access lines of a deck may be separately accessiblefrom the memory cells associated with access lines of a different deck.By connecting each memory cell to first and second access lines in a 3Dcross-point array, each memory cell 184 is individually accessible byspecifying the access lines to which the memory cell 184 is coupled, forexample, by a memory address. In a number of embodiments, memory array180 can include more or access bit lines 182 and 186, and/or memorycells than shown in the example of FIG. 1B.

FIG. 2 illustrates a portion of a memory according to an embodiment ofthe invention. FIG. 2 illustrates a voltage source 210 that providesvoltages to a decoder circuit 230. The decoder circuit 230 is coupled toaccess lines and memory cells of a memory array 228. For example, thedecoder circuit 230 is coupled to access lines 240(0) and 240(1) througha global access line GBL, and is configured to select access lines toaccess memory cells of the memory array 228. Memory cells 250(0) and250(1) are coupled to the access lines 240(0) and 240(1), respectively.The global access line GBL may be a global bit line and the access lines240(0) and 240(1) may be local bit lines. Access lines 244(0) and 244(1)are also coupled to the memory cells 250(0) and 250(1). The memory cells250(0) and 250(1) are coupled at the intersections between the accessline 244(0) and the access lines 240(0) and 240(1), respectively.Decoder circuit 260 is coupled to the access lines 244(0) and 244(1)through a global access line GWL. The global access line GWL may be aglobal word line and the access lines 244(0) and 244(1) may be localword lines.

Selection of the access lines 240 and 244 by the decoder circuits 230and 260 may be based on a memory address associated with a memory accesscommand. A sense amplifier 270 may be selectively coupled to the accesslines 244(0) or 244(1) to compare a voltage of the access line to areference voltage REF to determine a logic value of the data stored by amemory cell. The sense amplifier 270 is activated responsive to acontrol signal FSENB. During a memory access operation, a voltage source280 is configured to provide an access line voltage WLVDM to a selectedaccess line (e.g., access line 244(0), 244(1), etc.) responsive to acontrol signal WLVDM_EN. In some embodiments, the WLVDM voltage is anegative voltage, and the WLVDM voltage provides an increased voltageacross a target memory cell being accessed. Memory cells (e.g., memorycells 250(0) and 250(1)) may be written (e.g., set) to store data of afirst logic value. A memory cell may remain unwritten or may be erased(e.g., reset) to change the data stored by the memory cell to a secondlogic value. The data of a memory cell that is accessed is sensed byproviding an access voltage across a target cell through the respectiveaccess lines. Typically, set memory cells have a lower threshold voltagethan reset memory cells, and thus, a set memory cell is configured toconduct a current from one access line to the other access line when theaccess voltage across the set cell exceeds the lower threshold voltage.In contrast, a reset memory cell is configured to not conduct a currentfor the same voltage due to the higher threshold voltage.

The voltage source 210 includes a voltage circuit 212 having a sourcefollower circuit. The voltage circuit 212 includes transistors 214 and216. The voltage circuit 212 provides a voltage to a node AXN responsiveto control signals VDM0_SELB and VDM0. The voltage circuit 212 isconfigured to receive a supply voltage VHH, and the voltage provided bythe voltage circuit 212 may be based on the VHH voltage. When activatedby the VDM0_SELB signal, the transistor 214 provides the VHH voltage tothe drain of the transistor 216. The transistor 216 is configured toprovide a voltage to the AXN node based on the VDM0 signal and the VHHvoltage. For example, assuming the VDM0 signal is equal to VHH, theresulting voltage provided through the transistor 216 to the AXN node isVHH-Vt (216), where Vt (216) is the threshold voltage Vt of thetransistor 216. The voltage of the VDM0 signal may be adjusted to varythe voltage provided by the voltage circuit 212 to the AXN node.

The voltage source 210 further includes a voltage circuit 218 includinga source follower circuit having transistors 220 and 222. The voltagecircuit 218 provides a voltage to the AXN node responsive to controlsignals VDM1_SELB and VDM1. The voltage circuit 218 is configured toreceive a supply voltage VPP. When activated by the VDM1_SELB signal,the transistor 220 provides the VPP voltage to the drain of thetransistor 222. The transistor 222 is configured to provide a voltage tothe AXN node based on the VDM1 signal and the VPP voltage. For example,assuming the VDM1 signal is equal to VPP, the resulting voltage providedthrough the transistor 222 to the AXN node is VPP-Vt (222), where Vt(222) is a threshold voltage Vt of the transistor 222. In someembodiments, the supply voltage VPP is greater than the supply voltageVHH, however, in other embodiments the supply voltages VPP and VHH areequal, or the supply voltage VHH is greater than the supply voltage VPP.In some embodiments, the supply voltage VPP may be pumped voltage, thatis, a voltage that is greater than an externally provided supplyvoltage. A charge pump may be used to provide a pumped voltage. Thesupply voltage VPP and/or the voltage VPP-Vt may be greater than the Vtof a reset memory cell in some embodiments. In some embodiments, thesupply voltage VPP and/or the voltage VPP-Vt may be less than the Vt ofa reset memory cell. The supply voltage VPP and/or the voltage VPP-Vtmay be approximately equal to the Vt of a reset memory cell in otherembodiments.

A discharge circuit 224 is also included in the voltage source 210. Thedischarge circuit 224 includes a transistor that is coupled to the AXNnode. The discharge circuit 224 couples the AXN node to a referencevoltage (e.g., ground) responsive to a control signal AXN_PULL_DOWN.When activated, the discharge circuit 224 provides a conductive pathfrom the AXN node to the reference voltage to discharge a voltage on theAXN node.

In the embodiment of FIG. 2, the transistors 214 and 220 are shown asp-channel field effect transistors (pFETs) and the transistors 216 and222, and the discharge circuit 224 are shown as n-channel field effecttransistors (nFETs). In other embodiments, however, different types oftransistors may be used in the voltage source 210 without departing fromthe scope of the present invention.

The decoder circuit 230 includes a transistor 232 to couple the AXN nodeto the global access line GBL responsive to a control signal GBL_SELB.The decoder circuit further includes transistors 234(0) and 234(1)configured to selectively couple the GBL to respective access lines240(0) and 240(1) responsive to respective control signals LBL(0)_SELBand LBL(1)_SELB. A local signal line discharge circuit 236(0) isconfigured to couple the access line 240(0) to a reference voltage(e.g., ground) responsive to the LBL(0) SELB signal. For example,responsive to an inactive LBL(0)_SELB signal (e.g., LBL(0)_SELB having ahigh logic value), the local signal line discharge circuit 236(0) isactivated to couple the access line 240(0) to the reference voltage. Alocal signal line discharge circuit 236(1) is configured to couple theaccess line 240(1) to the reference voltage responsive to theLBL(1)_SELB signal. The local signal line discharge circuits 236(0) and236(1) may be activated to discharge the respective access lines afterthe decoder circuit 230 decouples the respective access line from theAXN node. The transistors 232, 234(0), and 234(l) are illustrated inFIG. 2 as pFETs, and the local signal line discharge circuits 236(0) and236(1) are shown as nFETs. However, different types of transistors maybe used in other embodiments without departing from the scope of thepresent invention.

The decoder circuit 260 includes a transistor 262(0) configured tocouple the access line 244(0) to the global access line GWL responsiveto a control signal LWL(0)_SEL, and further includes a transistor 262(1)configured to couple the access line 244(1) to the GWL responsive to acontrol signal LWL(1)_SEL. A transistor 264 is configured to couple theGWL to receive a WLVDM voltage (when the voltage source 280 is enabled)and to an input of the sense amplifier 270 when the transistor 264 isactivated responsive to a control signal GWL_SEL. When activated by theFSENB signal, the sense amplifier 270 provides an output signalSENSE_OUT having a logic value based on the voltage of the GWL relativeto the reference voltage REF provided to the sense amplifier 270.

It will be appreciated that FIG. 2 illustrates a portion of a memory,and that a memory may include additional circuits, access lines, memorycells, etc. illustrated by and described with reference to FIG. 2.Additionally, some or all of the various signals previously describedmay be provided by control logic and/or an I/O control circuit (e.g.,control logic 110 and/or I/O control circuit 120 of FIG. 1A).

In some embodiments, the memory cells 250 are phase change memory cells.In some embodiments, the memory cells 250 include chalcogenide material.In some embodiments, the memory cells 250 and the access lines arearranged as a cross point array.

As previously discussed, memory cells (e.g., memory cells 250(0) and250(1)) may be set to store data of a first logic value. A memory cellmay be reset to store data of a second logic value. The data of a memorycell that is accessed is sensed by providing an access voltage across atarget cell through the respective access lines. Typically, set memorycells have a lower threshold voltage than reset memory cells, and thus,a set memory cell is configured to conduct a current from one accessline to the other access line when the access voltage across the setcell exceeds the lower threshold voltage. In contrast, a reset memorycell is configured to not conduct a current for the same voltage due tothe higher threshold voltage. The current (or lack thereof) results in avoltage that is compared by the sense amplifier 270 against a referencevoltage REF to determine the logic value stored by the target memorycell. For example, a set memory cell may result in a voltage on the GWLthat is lower than the REF voltage, whereas a reset memory cell mayresult in a voltage on the GWL that is higher than the REF voltage.

A read operation of the memory cell 250(0) according to an embodiment ofthe invention will now be described with reference to FIG. 3. The memorycell 250(0) in the example read operation of FIG. 3 is assumed to beset. A read command and memory address is received by the memory. Thememory address for the memory cell 250(0) is decoded and thecorresponding access lines 240(0) and 244(0) are identified. ActiveGWL_SEL and LWL(0)_SEL signals (e.g., active high logic value) activatethe transistors 264 and 262(0) of the decoder circuit 260, and activeGBL_SELB and LBL(0)_SELB signals (e.g., active low logic value) activatethe transistors 232 and 234 of decoder circuit 230.

At time T0, the VDM0_SELB signal changes from a high logic value to alow logic value to activate the transistor 214 to provide the VHHvoltage to the AXN node through the transistor 216. It is assumed forthe example of FIG. 3 that the voltage of the VDM0 signal exceeds the Vtof the transistor 216 and is equal to VHH so that a voltage of VHH-Vt(216) is provided to the AXN node by the voltage circuit 212. It will beappreciated that the VDM0 signal may be other voltages as well and isnot limited to the specific voltage of the present example. Also at timeT0, the voltage source 280 is activated to provide the WLVDM voltage tothe memory cell 250(0) through the access line 244(0). As a result, thevoltage across the memory cell 250(0), shown in FIG. 3 as LBL-LWL,increases.

At time T1, the VDM1_SELB signal changes from a high logic value to alow logic value to activate the transistor 220 to provide the VPPvoltage to the AXN node through the transistor 222. It is assumed forthe example of FIG. 3 that the voltage of the VDM1 signal exceeds the Vtof the transistor 222 and is equal to VPP so that a voltage of VPP-Vt(222) is provided to the AXN node by the voltage circuit 218. Thevoltage of the VDM1 signal may be adjusted to vary the voltage providedto the AXN node. Thus, following time T1, both the voltage circuits 212and 218 are activated to provide a respective voltage to the AXN node.

As previously discussed, it is assumed for the example of FIG. 3 thatthe memory cell 250(0) is set, thus, as the voltage across the memorycell 250(0) continues to increase, a threshold voltage of the set memorycell 250(0) is exceeded, and the memory cell 250(0) begins to conductcurrent. As a result, the current conducted through the memory cell250(0) from the access line 240(0) to the access line 244(0) causes thevoltage of the access line 244(0) to increase from the WLVDM voltage.

At time T2, the VDM1_SELB signal changes from a low logic value to ahigh logic value to deactivate the transistor 220 thereby deactivatingthe voltage circuit 218 from providing a voltage to the AXN node. TheVDM0_SELB signal remains at the low logic value, however, so that thevoltage circuit 212 continues to provide a voltage to the AXN node. Attime T3, the FSENB signal changes from a high logic value to a low logicvalue to activate the sense amplifier 270 to compare the voltage of theaccess line 244(0) to the REF voltage. With the increased voltage of theaccess line 244 caused by the current conducting through the memory cell250(0), the sense amplifier 270 provides a SENSE_OUT signal indicatingthat the memory cell 250(0) is set (e.g., the voltage of the access line244 is greater than the REF voltage). Also at time T3, the AXN_PULL_DOWNsignal changes from a low logic level to a high logic level to activatethe discharge circuit 224. The activated discharge circuit 224 providesa discharge path from the AXN node to the reference voltage to dischargevoltage that exceeds the voltage provided by the voltage circuit 212,which continues to be activated. Thus, at least for a portion of whenthe voltage circuit 212 provides a voltage to the AXN node the activateddischarge circuit 224 discharges the AXN node.

At time T4, the FSENB signal changes from the low logic value to thehigh logic value to deactivate the sense amplifier 270. Also at time T4,the VDM0_SELB signal changes from the low logic value to a high logicvalue to deactivate the transistor 214 thereby deactivating the voltagecircuit 212 from providing a voltage to the AXN node. The AXN node maybe discharged through the discharge circuit 224 to the referencevoltage. At time T5, the AXN_PULL_DOWN signal changes from the highlogic level to the low logic level to deactivate the discharge circuit224.

A read operation of the memory cell 250(0) according to an embodiment ofthe invention will now be described with reference to FIG. 4. The memorycell 250(0) accessed in the read operation of FIG. 4 is assumed to bereset. A read command and memory address is received by the memory. Thememory address is decoded and the corresponding access lines 240(0) and244(0) are identified. The GWL_SEL signal and the LWL(0)_SEL signalsactivate the transistors 264 and 262(0) of the decoder circuit 260, andthe GBL_SELB and LBL(0)_SELB signals activate the transistors 232 and234 of decoder circuit 230.

At time T0, the VDM0_SELB signal changes from a high logic value to alow logic value to activate the transistor 214 to provide the VHHvoltage to the AXN node through the transistor 216. It is assumed forthe example of FIG. 4 that the voltage of the VDM0 signal exceeds the Vtof the transistor 216 so that a voltage of VHH-Vt (216) is provided tothe AXN node by the voltage circuit 212. Also at time T0, the voltagesource 280 is activated to provide the WLVDM voltage to the memory cell250(0) through the access line 244(0). As a result, the voltage acrossthe memory cell 250(0), shown in FIG. 4 as LBL-LWL, increases.

At time T1, the VDM1_SELB signal changes from a high logic value to alow logic value to activate the transistor 220 to provide the VPPvoltage to the AXN node through the transistor 222. It is assumed forthe example of FIG. 4 that the voltage of the VDM1 signal exceeds the Vtof the transistor 222 so that a voltage of VPP-Vt (222) is provided tothe AXN node by the voltage circuit 218. Thus, following time T1, boththe voltage circuits 212 and 218 are activated to provide a respectivevoltage to the AXN node.

As previously discussed, it is assumed for the example of FIG. 4 thatthe memory cell 250(0) is reset, thus, in contrast to the set cell ofFIG. 3, the threshold voltage of the reset memory cell 250(0) is notexceeded and the voltage across the memory cell 250(0) continues toincrease. At time T2, the VDM1_SELB signal changes from a low logicvalue to a high logic value to deactivate the transistor 220 therebydeactivating the voltage circuit 218 from providing a voltage to the AXNnode. The VDM0_SELB signal remains at the low logic value, however, sothat the voltage circuit 212 continues to provide a voltage to the AXNnode. Deactivating the voltage circuit 218 prevents the voltage across amemory cell from continuing to increase when the memory cell is reset.In some embodiments, the voltage circuit 218 is deactivated to stopproviding a voltage to the AXN node before the voltage circuit 212 isdeactivated to stop providing a voltage to the AXN node. In someembodiments, the voltage circuit 218 is deactivated to stop providing avoltage before the AXN node reaches the voltage of the voltage circuit218. That is, the voltage of the AXN node increases over time, and thevoltage circuit 218 is deactivated before sufficient time for the AXNnode to reach the voltage provided by the voltage circuit 218. Thehigher voltage provided by the voltage circuit 218 may degrade the resetstate of a memory cell is the voltage across the memory cell is allowedto continue to increase, and if repeated over time may cause the memorycell to have an indeterminate state. Thus, deactivating the voltagecircuit 218 may help maintain the integrity of the reset state of thememory cell. The timing of activation and deactivation of the voltagecircuits 212 and 218 may be set based on design and/or performanceparameters for a memory device. For example, the timing may be setduring testing of the memory device, such as through the use of fuse orantifuse circuits.

At time T3, the FSENB signal changes from a high logic value to a lowlogic value to activate the sense amplifier 270 to compare the voltageof the access line 244 to the REF voltage. The voltage of the accessline 244 remains approximately at the WLVDM voltage because the resetmemory cell 250(0) conducts little to no current. As a result the senseamplifier 270 provides a SENSE_OUT signal indicating that the memorycell 250(0) is reset (e.g., the voltage of the access line 244 is lessthan the REF voltage). Also at time T3, the AXN_PULL_DOWN signal changesfrom a low logic level to a high logic level to activate the dischargecircuit 224. The activated discharge circuit 224 provides a dischargepath from the AXN node to the reference voltage to discharge voltage inexcess to that provided by the voltage circuit 212, which continues tobe activated.

At time T4, the FSENB signal changes from the low logic value to thehigh logic value to deactivate the sense amplifier 270. Also at time T4,the VDM0_SELB signal changes from the low logic value to a high logicvalue to deactivate the transistor 214 thereby deactivating the voltagecircuit 212 from providing a voltage to the AXN node. The AXN node maybe discharged through the discharge circuit 224 to the referencevoltage. At time T5, the AXN_PULL_DOWN signal changes from the highlogic level to the low logic level to deactivate the discharge circuit224.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

What is claimed is:
 1. An apparatus comprising: a memory comprisingcontrol circuitry and a memory array comprising a plurality of memorycells, wherein during a memory access operation associated with a memorycell of the plurality of memory cells, causing a first voltage to beprovided to the memory cell during a first time period and causing asecond voltage to be provided to the memory cell during a second timeperiod, wherein the second time period overlaps with the first timeperiod and has a shorter duration than the first time period.
 2. Theapparatus of claim 1, wherein the first voltage is different than thesecond voltage.
 3. The apparatus of claim 2, wherein the first voltageis less than the second voltage.
 4. The apparatus of claim 1, whereinthe control circuitry is configured to cause the first voltage to beprovided to the memory cell prior to causing the second voltage to beprovided to the memory cell.
 5. The apparatus of claim 4, wherein thecontrol circuitry is configured to cause the second voltage to stopbeing provided to the memory cell after the second time period andduring the first time period.
 6. The apparatus of claim 1, wherein thecontrol circuitry comprises at least one of: an input/output (I/O)control circuit configured to receive I/O signals that includeinformation associated with the memory access operation; or controllogic configured to receive control signals including informationassociated with the memory access operation.
 7. The apparatus of claim1, wherein the control circuitry is further configured to cause a senseamplifier to sense data stored at the memory cell.
 8. The apparatus ofclaim 1 further comprising a source follower circuit configured toprovide the first voltage.
 9. The apparatus of claim 1, wherein thecontrol circuit is configured to cause a node receiving the secondvoltage to discharge after the second time period and during the firsttime period.
 10. The apparatus of claim 1, wherein the memory cellcomprises a phase change memory cell.
 11. A method, comprising: during amemory access operation associated with a memory cell of a plurality ofmemory cells of a memory: causing, via a control circuit of the memory,a first voltage to be provided to the memory cell during a first timeperiod; and causing a second voltage to be provided to the memory cellduring a second time period, wherein the second time period overlapswith the first time period and has a shorter duration than the firsttime period during a read operation of a target cell:
 12. The method ofclaim 11, wherein the first voltage is different than the secondvoltage.
 13. The method of claim 12, wherein the first voltage is lessthan the second voltage.
 14. The method of claim 11, further comprisingcausing the first voltage to be provided to the memory cell prior tocausing the second voltage to be provided to the memory cell.
 15. Themethod of claim 14, further comprising causing the second voltage tostop being provided to the memory cell while causing the first voltageto be provided to the memory cell.
 16. The method of claim 11 furthercomprising causing the first voltage to be provided via a sourcefollower circuit.
 17. The method of claim 11, further comprising causinga node receiving the second voltage to discharge after the second timeperiod and during the first time period.
 18. The method of claim 11,further comprising causing a sense amplifier to sense data stored at thememory cell.
 19. The method of claim 18, wherein causing the senseamplifier to sense data stored at the memory cell is during the firsttime period.
 20. The method of claim 19, wherein causing the senseamplifier to sense data stored at the memory cell is after the secondtime period.